This invention relates generally to circuitry for use with a capacitive sensor, and more particularly for use with a micromachined accelerometer.
Micromachined accelerometers can be used to sense acceleration for a variety of applications, including sensing the acceleration that occurs as a result of an automobile accident in order to trigger an air bag, or sensing the acceleration resulting from an earthquake in order to automatically shut off a gas line to prevent fires. One type of micromachining process is referred to as surface micromachining, a process by which a sensor structure is formed in layers over a substrate using semiconductor processing techniques such as depositing and etching. U.S. Pat. No. 5,326,726 describes such a process and is incorporated herein by reference in its entirety for all purposes.
In one type of micromachined device currently made by the assignee of the present invention, a polysilicon mass is suspended over a substrate by supporting tethers. The mass, which is essentially parallel to the substrate, has a beam elongated along an axis, and a number of fingers that extend away from the beam in a direction perpendicular to the axis of the beam. The beam and fingers are movable laterally relative to the substrate along the axis. Each of these movable fingers is positioned between two polysilicon fingers that are in the plane of the mass and are fixed relative to the substrate. Each movable finger and the fixed fingers on either side of the movable finger form a differential capacitor cell. The cells additively form a differential capacitor. A structure of this type is shown, for example, in U.S. Pat. No. 5,345,824, which is incorporated herein by reference in its entirety and for all purposes.
Different approaches can be used to sense acceleration with such a differential capacitor. One approach is to use force feedback, as described in U.S. Pat. No. 5,345,824. The movable fingers (i.e., movable with the mass) are each centered between two fixed fingers. All the fixed fingers on one side of the movable fingers are electrically coupled together, and all the fixed fingers on the other side of the movable fleers are also electrically coupled together. The two sets of fixed fingers are at different DC potentials and are driven with AC carrier signals that are 180xc2x0 out of phase with respect to each other.
In response to an external force/acceleration along a sensitive axis, the mass with movable fingers moves toward one or the other set of fixed fingers. The signal on the beam is amplified, demodulated, and provided to an output terminal. A feedback network connects the output terminal and the beam. The feedback causes the movable fingers to be re-centered between the two sets of fixed fleers. The signal at the output terminal is a measure of the force required to re-center the beam, and is therefore proportional to acceleration.
One alternative to this closed-loop force feedback circuit is an open-loop circuit. As shown in prior art FIG. 1, a sensor cell 10 has a movable electrode 12 between a first electrode 14 and a second electrode 16. As shown also in U.S. Pat. No. 5,659,262, which is expressly incorporated herein by reference in its entirety and for all purposes, electrodes 14 and 16 are driven by respective drivers 18 and 20. Each driver provides a 100 kHz square wave that alternates between two voltages, e.g., 0 volts and 5 volts. The signals from the drivers are 180xc2x0 out of phase, such that one set of finger is at 0 volts while the other is at 5 volts.
In response to an acceleration, the beam moves toward one set of electrodes 14, 16, causing an AC output signal to appear on the beam. This signal is a square wave signal that is in phase with the driver signal of whichever electrode 14, 16 toward which electrode 12 moves, and has an amplitude on the order of millivolts. The amplitude is approximated proportional to acceleration for small displacements. The beam signal provided to an amplifier 22 and a demodulator 26 produces an output signal in terms of V/g at an output terminal 28, thereby indicating the acceleration.
In this open loop design, because the acceleration is determined from the magnitude of the output signal, it is important that the signal processing circuitry that provides a signal to an output terminal (e.g., an amplifier and demodulator) be precise. For example, the amplifier should have a precise gain, and the circuitry should be insensitive to temperature drift and other factors that can affect the output.
The force feedback described earlier design reduces the need for precise circuitry and substantially reduces problems that can arise due to parasitic capacitive effects. But force feedback has other drawbacks: it does not have ratiometric operation, and a dc bias must usually be applied to the sensor, giving rise to charge-induced offset and other undesirable effects. Because the mechanical transfer function is inside the loop, there can be problems with the stability of the loop.
The open loop design has drawbacks as well. Typically, the scale factor is dependent on the parasitic capacitance of the movable electrode and the circuits connected to it. This capacitance includes junction capacitances that vary with voltage and temperature causing scale factor variation. The desired ratiometric behavior is affected, as is the temperature coefficient of scale factor. The drive signals on the first and third electrodes also create electrostatic forces that vary with the position of the second electrode, effectively altering the mechanical response of the sensor. Since these electrostatic forces change with supply voltage, the output is not strictly ratiometric.
The present invention is a closed loop electromechanical system with a feedback method that electrically rebalances an output signal of a sensor under the force of acceleration without applying a significant force to the sensor. The system has a movable component that is movable relative to another component. This feedback is accomplished by unbalancing clock signals applied to some components of the sensor so as to electrically null the output on the movable component in a manner that does not appreciably change the forces on the movable component. This feedback provides the benefits of closed loop operation, without the use of force feedback, and minimizes the effects of electrostatic forces on scale factor.
The invention includes feedback and driving circuitry, a sensor with feedback and driving circuitry, and methods for performing sensing with a micromachined sensor of the differential capacitor type. The sensor has a movable mass suspended over a substrate and movable relative to the substrate in a manner similar to the aforementioned sensors. The
In one aspect, the invention includes a sensor with first, second, and third electrodes with the second electrode movable relative to the first and third electrodes to form a differential capacitor, first and second drivers for providing drive signals to the first and third electrodes, signal processing circuitry coupled between the second electrode and an output terminal, and a feedback circuit coupled between the output terminal and at least the first driver to control the drive signal amplitude to the first electrode. The drive signals from the first and second drivers are preferably square waves, with one drive signal 180xc2x0 out of phase with the other drive signal. The circuitry from the second electrode to the output terminal preferably includes an amplifier and a demodulator.
The feedback adjusts the amplitude of one (or preferably both) drivers to null the signal on the second electrode. The feedback causes the amplitude of one or both drive signals to be adjusted, preferably in such a way that there is substantially no AC force and no change in the static force on the second electrode. The adjustment of the drivers that nulls the second electrode signal has a precise relationship to the motion of the second electrode and the output signal is determined precisely from the amount of adjustment. As a consequence of nulling, the effects of electrostatic forces on scale factor are significantly minimized.
The electrodes are preferably part of a surface micromachined accelerometer that has a movable mass suspended over the substrate and having a movable beam and rows of fingers (in total constituting a second electrode). The first and third electrodes are fingers fixed relative to the substrate and on either side of the fingers extending from the movable beam. The fixed electrodes are driven with high frequency carrier signals that are opposite in phase. The accelerometer can have a single mass movable along one axis, two or more masses, or one or more masses movable along multiple axes.
The invention also includes a method for sensing acceleration with a capacitive sensor that has a first electrode, a second electrode, and a third electrode, with the second electrode being between and movable relative to the first and third electrodes to form a differential capacitor, and drivers for providing drive signals to the first and third electrodes. The method includes processing a signal on the second electrode and providing feedback to one or preferably both of the drivers, preferably to null the AC signal on the second electrode without creating an AC force or changing the electrostatic forces on the second electrode.
The invention also includes a micromachined sensor with a substrate, and first, second, and third electrodes suspended over the substrate with the second electrode movable with respect to the first and third electrode. Circuitry that is preferably integrated into the same substrate as the sensor includes drivers for providing signals to the first and third electrodes, circuitry for processing a signal on the second electrode and for providing a signal from the second electrode to an output terminal, and feedback circuitry between the output terminal and the driver to control the signal from the driver. The drivers provide to the first and third electrode periodic signals, preferably square waves, 180xc2x0 out of phase to each other. The feedback signal controls the drivers to change the amplitude of at least one of the periodic signals, preferably to null the signal on the second electrode without creating an AC force or changing the static forces on the second electrode.
The circuitry of the present invention can take a number of different forms. For example, the circuitry can include one or two opamps for receiving the signal from the movable beam. In addition, there are embodiments with one or two differential pairs of transistors, and embodiments with no opamps or transistors. Another embodiment combines the amplifying and demodulating circuitry with driver circuitry. These different circuits have different benefits and drawbacks, such as accuracy, sensitivity, the ability to change sensitivity, space, and number of components. The embodiments of the circuits have in common the ability to adjust the amplitude of at least one periodic signal provided to an electrode in response to an input signal from another electrode.
The sensor and circuitry of the present invention have a number of benefits. The system has the advantages of a closed feedback network, and can be made ratiometric (the scale factor of volts to g""s changes in proportion to the power supply voltage), independent of mechanical forces, and avoids the need for a DC bias relative to the drive signals on the beam. The feedback is provided to at least one driver, and preferably is provided to each driver, to null the signal on a movable beam so that the signal has no AC component in response to a sensed acceleration. With substantially no such AC component, parasitic capacitances on the movable electrode have minimal effect on the resulting output signal. With this system, less precision is required in the processing circuitry compared to other designs in that the amplifier need not have a precise gain, and the gain of the circuitry need not be insensitive to temperature. Consequently, the processing circuitry can be simplified relative to the processing circuitry used, for example, in an open-loop design. Other features and advantages will become apparent from the following detailed description, drawings and claims.